1. Field
Embodiments relate to a pixel and an organic light emitting display device, and more specifically, to a pixel that is useful for implementing high resolution and high frequencies, and an organic light emitting display device using the pixel.
2. Description of the Related Technology
Various flat panel display devices that have advantages over cathode ray tubes, that is, the weight and size, have been developed. Flat panel display devices, liquid crystal display devices, field emission display devices, plasma display panels, and organic light emitting display devices have been proposed.
The organic light emitting display devices display images using OLEDs (organic light emitting diodes) that generate light by recombination of electrons and holes.
The field of application of such organic light emitting display devices has increasingly expanded to include PDAs, MP3 players, mobile phones, etc. due to various advantages, such as high color reproduction and a small thickness.
The organic light emitting diodes used for the organic light emitting display devices include an anode electrode, a cathode electrode, and a light emitting layer formed therebetween. The organic light emitting diode emits light, when electric current flows from the anode electrode and the cathode electrode, and the amount of emitted light changes in accordance with changes in the amount of electric current, such that luminance is controlled.
FIG. 1 is a circuit diagram illustrating a pixel employed in some light emitting display devices. Referring to FIG. 1, the pixel includes an organic light emitting diode OLED, a first transistor T1, a second transistor T2, a third transistor T3, a fourth transistor T4, a fifth transistor T5, a sixth transistor T6, and a capacitor Cst. The first to sixth transistors T1 to T6 have a gate electrode, a source electrode, and a drain electrode, and the capacitor Cst is composed of a first electrode and a second electrode.
Regarding the first transistor T1, the source electrode is connected to a first node A, the drain electrode is connected to a second node B, and the gate electrode is connected to a third node C.
Regarding the second transistor T2, the source electrode is connected to a data line Dm and the drain electrode is connected to the first node A. Further, the gate electrode is connected to a first scanning line Sn. Therefore, a data signal is transmitted to the first node A by a first scanning signal that is inputted through the first scanning line Sn.
Regarding the third transistor T3, the source electrode is connected to the second node B, the drain electrode is connected to the third node C, and the gate electrode is connected to the first scanning line Sn. The potentials of the second node B and the third node C become the same, when the third transistor T3 is turned on by the first scanning signal that is transmitted through the first scanning line.
Regarding the fourth transistor T4, the source is connected to an initialization power supply VINT, the drain electrode is connected to the third node C, and the gate electrode is connected to a second scanning line Sn−1. In this configuration, a scanning signal that is transmitted to the second scanning line Sn−1 is a scanning signal that allows a data signal to be transmitted to the pixel of the previous row.
Regarding the fifth transistor T5, the source electrode is connected to a first pixel power supply ELVDD, the drain electrode is connected to the first node A, and the gate electrode is connected to a light emitting control line En. Therefore, the first pixel power supply ELVDD is selectively supplied to the first transistor T1, in accordance with a light emitting control signal that is transmitted through the light emitting control line.
Regarding the sixth switching transistor T6, the source electrode is connected to the third node C, the drain electrode is connected to the organic light emitting diode OLED, and the gate electrode is connected to the light emitting control line En. Therefore, the electric current flowing from the source electrode of the first transistor to the drain electrode is selectively transmitted to the organic light emitting diode OLED, in accordance with the light emitting control signal that is transmitted through the light emitting control line En.
Regarding the capacitor Cst, the first electrode is connected to the first pixel power supply ELVDD and the second electrode is connected to the third node C. Therefore, when an initialization signal is transmitted to the third node C by the fourth transistor T4, the capacitor Cst stores the initialization voltage so that the third node C maintains the initialization voltage. Further, when a data signal is transmitted to the first transistor T1 by the second transistor T2 and the third transistor T3, the third node C stores voltage corresponding to the data signal.
The current in the OLED is represented by the following Formula 1:
                              I          OLED                =                              β            2                    ⁢                                    (                                                V                  ⁢                  gs                                -                                  V                  ⁢                  th                                            )                                      2              =                                ⁢                      β            2                    ⁢                                          ⁢                                    (                              Vdata                -                                  EL                  ⁢                  V                  ⁢                  DD                                +                                  V                  ⁢                  th                                -                                  V                  ⁢                  th                                            )                                      2              =                                ⁢                      β            2                    ⁢                                    (                                                V                  ⁢                  data                                -                                  EL                  ⁢                  V                  ⁢                  DD                                            )                        2                                              [                  Formula          ⁢                                          ⁢          1                ]            where IOLED is electric current flowing to the organic light emitting diode OLED, Vgs is voltage applied in between the gate electrode and the source electrode of the first transistor T1, EVDDD is voltage of the first pixel power supply, Vth is threshold voltage of the first transistor T1, and Vdata is voltage of a data signal.
Referring to Formula 1, the electric current flowing to the organic light emitting diode OLED by the first transistor corresponds to the voltage of the data signal and the voltage of the first pixel power ELVDD, and is independent of the threshold voltage Vth of the first transistor. Therefore, the threshold voltage is compensated.
However, since the electric current flows according to the voltage of the first pixel power ELVDD and the data signal, and the first pixel power transmitted to each pixel is different by voltage drop, uniform electric current does not flow to the pixels.
Further, when the organic light emitting display device has high resolution and receives a high-frequency driving signal, the length of one horizontal time is reduced. For example, the organic light emitting display device is driven at FHD resolution and 60 Hz, the length of one horizontal time is 14.8 μs, while it is driven at FHD resolution and 120 Hz, the length of one horizontal time decreases to 7.4 μs.
As the length of one horizontal time reduces, the time for compensating the threshold voltage reduces, such that the picture quality is deteriorated.